The formation of various integrated circuit (IC) structures often relies on photolithographic processes, or photolithography. For instance, a protective mask arrangement can be formed from a photo resist (PR) layer by passing light energy through a mask (or reticle) having a pattern corresponding to the desired arrangement to expose the PR layer. As a result, the pattern of the reticle is transferred to the PR layer. In areas where the PR is sufficiently exposed and after a development cycle, the PR material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, etc.). Portions of the PR layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer. The exposed portions of the underlying layer can then be etched (e.g., by using a chemical wet etch or a dry reactive ion etch (RIE)) such that the pattern formed from the PR layer is transferred to the underlying layer. Alternatively, the PR layer can be used to block dopant implantation into the protected portions of the underlying layer or retard reaction of the protected portions of the underlying layer. Thereafter, the remaining portions of the PR layer can be stripped.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. For example, line widths and the separation between lines is becoming increasingly smaller. For example, nodes with a critical dimension of about 45 nanometers (nm) to about 65 nm have been proposed. In these sub-micron processes, silicon yield is affected by factors such as reticle/mask pattern fidelity, optical proximity effects and PR processing. Some of the more prevalent concerns include line end pullback, corner rounding and line-width variations. These concerns are largely dependent on local pattern density and topology.
Optical proximity correction (OPC) has been used to improve image fidelity. In general, current OPC techniques involve running a computer simulation that takes an initial data set having information relating the desired pattern and manipulates the data set to arrive at a corrected data set in an attempt to compensate for the above-mentioned concerns. Briefly, the OPC process can be governed by a set of optical rules (i.e., “rule-based OPC” employing fixed rules for geometric manipulation of the data set), a set of modeling principles (i.e., “model-based OPC” employing predetermined behavior data to drive geometric manipulation of the data set) or a hybrid combination of rule-based OPC and model-based OPC.
The computer simulation can involve iteratively refining the data set using an edge placement error value as a benchmark for the compensating process. That is, the data set is manipulated based on the rules and/or models and the predicted placement of the edges contained in the pattern are compared against their desired placement. For each edge, or segment thereof depending on how the edges are fragmented in the data set, a determination of how far the predicted edge/segment placement deviates from the desired location is derived. For instance, if the predicted edge placement corresponds to the desired location, the edge placement error for that edge will be zero. As the predicted edge placement varies from the desired location, a positive or negative value in nanometers (or fractions thereof) can be derived. The placement error values for each edge/segment are aggregated to derive a single edge placement error value for the iteration of the OPC simulation. In some OPC routines, when the edge placement error for the iteration falls below a predetermined threshold, a corrected data set is output by the computerized OPC simulation and that corrected data set is used in fabrication of the reticle.
Current OPC techniques work fairly well when the critical dimension is relatively large (e.g., 0.25 microns and larger). That is, using OPC with edge placement error as the driving factor, the corrected data set can become highly tuned. However, the Applicants have found that as IC structures become smaller, correction of the pattern data set using conventional techniques can lead to unexpected instability in the corrected data set if one or more process factors (e.g., focus, exposure dose or other illumination condition) were to change. This instability can result even if each process factor remains within its respective tolerances.
For example, using a set of given process factors (e.g., a certain focus, a certain exposure dose, etc.), the OPC process may position various points (or edge segments) to be within a given distance from corresponding desired locations using the edge placement error value. However, during actual illumination, a process factor could become slightly altered (e.g., focus could change by a few percent) that causes one or more of the corrected points to move more than an acceptable amount. In an simplified example, three points (named herein as points A, B and C) in the data pattern may be corrected and, under most process variations, will fall within a given level of acceptability to a desired location when imaged through the reticle. However, as a result of the OPC correction process making corrections to each of the points A, B and C, one point (for instance, point C) could become overly sensitive to one or more process factors, such as focus and/or expose dose. As the imaging system (e.g., a stepper) moves to expose various areas on a wafer, each process factor may change slightly. Therefore, during a particular exposure of the wafer to expose points A, B and C, the light pattern for point C could place point C at a relatively distant and unacceptable position from its desired location (e.g., about 15 nm from the desired location) due to a change in the process factor for which point C has become overly sensitive. In fact, some points could become so sensitive to certain process factors that the OPC process may introduce a larger error than if the OPC process had not been carried out.
Accordingly, there exists a need in the art for an improved methodology for simulated correction of a photolithographic pattern.